The present invention relates to an improved substrate and to an optical recording medium and an information record overlying such an improved substrate.
Information may be recorded by exposing a region of an optical recording medium to a recording light beam thereby changing the local optical properties of the exposed region. The simplest such recording medium is a monolayer structure having a light absorptive layer overlying a substrate. Recording of information is accomplished by locally melting or ablating, or altering the crystallinity of the absorptive layer to change the reflectivity of the exposed region.
Spong, in U.S. Pat. No. 4,097,895, entitled MULTILAYER OPTICAL RECORD and which is incorporated herein by reference, discloses a bilayer optical recording medium which comprises a light reflective layer coated with a light absorptive layer. The coating parameters for the light absorptive layer are chosen to establish a condition for reducing reflectivity for the coated record blank at the recording light frequency. Bell, in U.S. Pat. No. 4,216,501, entitled OPTICAL ANTI-REFLECTIVE INFORMATION RECORD and which is incorporated herein by reference, discloses a trilayer optical recording medium having a transparent spacer layer interposed between the reflective and absorptive layers of the bilayer recording medium. The spacer layer serves to provide an even more efficient structure for optical recording.
The optical recording media described above, however, do have the disadvantage that they are expensive. The glass or other substrate on which the reflective, spacer, and absorptive layers are deposited must have a high degree of surface perfection, since defects on a micron and even sub-micron scale cause signal dropouts or disturbances to the retrieved signal.
Corrections to some extent can be made for macroscopic defects in disc substrates. Dropout signal compensator methods can be utilized, but only when the magnitude of the perturbation goes beyond the normal range of video signal levels which correspond to picture information. These schemes are typically of less consequence in data storage applications. Disc runout, or deviation from flatness, can also be compensated for by means of focus servos to keep the objective lens in focus at a fixed distance from the recording surface. The sensitivity limitations of these servos hamper the effectiveness of this compensation method when dealing with sharp deviations from flatness.
Microscopic imperfections in the substrate heretofore were eliminated by careful mechanical polishing of a glass disc substrate surface. While polishing the glass proved to be a successful method of attaining the high surface quality desired, the glass disc substrate becomes less desirable when investigating a cost-effective program for practical mass production of high quality substrates. Polishing steps are costly. Further, glass is a very brittle material subject to breakage, particularly when spinning at high speeds, typically 1800 rpm, during recording and readout. Not only is the disc destroyed and time, materials, and recorded information lost, but some danger to the operator is apparent.
Polyvinylchloride (PVC) and polymethylmethacrylate (PMMA) can also be used as substrates for optical recording media and information records; they are safer than glass substrates and yet they are rigid enough to support the overlying structures. However, PVC and PMMA, even after the most careful preparations, do not have a surface suitable for high quality optical recording. Thus, these substrates are coated with a liquid nonconformal coating layer such as acrylic latex floor finishes, commercial lacquers and varnishes and the like, to help fill in surface defects and yield a smoother surface.
Applying this coating in a sufficient thickness to produce a high quality surface has been a problem, however. For example, an acrylic latex floor finish at a thickness of 2 to 3 microns results in a glossy, hard surface. Yet coating thicknesses of 5 to 10 microns and above are needed on most PVC substrates to produce a surface smooth enough for high quality optical recording. Applying acrylic solutions at thicknesses of this magnitude produces a foggy, undesirable, matte finish, due in part, at least, to an inability of this thicker material to cure properly. Multiple applications of thinner layers of acrylic latex yield similar problems. The deposition of subsequent layers that contain solvents attacks the preceding layers and less than optically smooth surfaces result.
Thus, an optically smooth substrate, free of macroscopic and microscopic imperfections and suitable for high quality optical recording and information storage has been sought.